Soybeans completely lacking lipoxygenase (hereinafter referred to as lipoxygenase-free soybeans) generate almost none of the bean odor peculiar to soybeans, and can prevent the production of lipid peroxide. The powder of lipoxygenase-free soybeans is advantageously used as soybean food material which is blended with other food material, such as wheat flour, as disclosed in, for example, Japanese Patent No. 2500350, “Method for processing soybean-processed food”. Unfortunately, soybeans contain various types of constituents for flavor and taste irrespective of whether they are good or not, and source constituents of disagreeable odor and taste cannot be completely removed, even from lipoxygenase-free soybeans.
For example, a process of soybean milk production leaves a strong taste (disagreeable taste or dry mouth feel) peculiar to raw bean curd.
Normal soybeans, not species free of lipoxygenase, generate such disagreeable odor and taste more strongly. Accordingly, the removal of such odor and taste has been studied. Exemplary approaches include:    (1) physical processes, such as heat or solvent treatment;    (2) chemical processes using a chemical agent;    (3) processes for masking with other flavors or tastes; and    (4) processes for separating protein.Among these approaches, heat treatment has been widely studied because it is relatively easy to perform. Disagreeable odors from normal soybeans are generated mainly by oxidation of soybean lipid induced by the enzyme activity of lipoxygenase, and the reaction product n-hexanal or the like is probably the causative substance of the disagreeable odor. Heat treatment is generally intended to eliminate the activity of lipoxygenase and to remove any causative substance that has already been produced. In this instance, high-temperature air or high-temperature water vapor is used as a heating medium. Special care should be taken in use of high-temperature air because it is likely to cause lipid oxidation and a side reaction which may brown the protein. On the other hand, water vapor has such a high heat-transfer efficiency as to be suitable as the heating medium.
Unfortunately, use of high-temperature water vapor of 100° C. or more requires expensive pressure-resistant equipment and makes the operation complicated.
Treatment under high-pressure conditions inevitably causes water vapor to penetrate soybeans, and thus requires an additional drying step. Consequently, the price of the product increases disadvantageously.
Soybean fine powder (having a particle size, for example, between several micrometers and several hundred micrometers) thrown into water floats on the surface of the water without sinking below the surface, due to the high surface tension. Even if the water is forcibly stirred, the powder is divided into several lumps and the lumps keep floating. In order to uniformly disperse the particles of the powder (hereinafter referred to as fine particles), stirring must be continued for a long time.
In order to solve this problem, the powder is generally subjected to granulation, in which fine particles are bound to each other with an appropriate force and form grains (hereinafter referred to as granules) having apparently larger sizes to the extent that the grains can be treated as powder. Consequently, an indefinitely large number of voids are formed between the fine particles except the binding sites. Thus, water coming into contact with the granules can penetrate the voids due to capillary action, so that the granules are rapidly impregnated with the water and sink easily.
While the granules are sinking, the appropriate bindings between the fine particles are released, so that the fine particles are uniformly dispersed to form a state that can be easily mixed with other food material.
For granulation of the powder, a variety of processes have been known, such as (1) rolling granulation, (2) extruding granulation, (3) compressing granulation, (4) pulverizing granulation, (5) stirring granulation, (6) fluidized-bed granulation, (7) spraying granulation, (8) melting granulation, (9) coating granulation, and (10) encapsulating granulation.
While it goes without saying that an appropriate granulation process is selected according to the properties of the powder material and desired products (granules), broadly adopted processes are performed such that mechanical force is appropriately applied by, for example, compression, shock, or shearing so that fine particles are bound to each other to form granules, as in the processes listed above.
In granulation of soybean powder with mechanical force to form soybean granules, oil naturally contained particularly in so-called whole soybean powder is squeezed by applying an excessive mechanical force, which consequently impairs the properties of the powder. It is therefore necessary to reduce the mechanical force as much as possible. This, however, undesirably provides granules having insufficient binding force between fine particles and inferior shape stability. Accordingly, a binder is added to enhance the binding force. Unfortunately, the binder degrades the purity of the resulting soybean product and may change the taste.
In view of the above-described circumstances, the object of the present invention is to provide a method for producing soybean powder free from disagreeable odor and taste and dispersible in water in a short process with inexpensive equipment, and a method for producing soybean milk.